Automotive engineers have long sought ways to improve vehicle handling and dynamic stability on low-friction road surfaces and under extreme steering inputs, for example during emergency situations. Until very recently, this technology has been limited by drive train technologies designed around a single power source, such as an internal combustion engine or electric motor.
One of the parameters that must be taken into consideration in automotive design is wheel slip, which is the difference between the velocity of a vehicle over a road and the velocity at which the outer edge of a tire mounted upon a wheel of the vehicle is moving. The terms “wheel” and “tire” are used interchangeably herein to refer to the combination of the tire mounted upon the wheel. Wheel slip is often expressed as a ratio (the “wheel slip ratio”), representing the difference of wheel and road velocities to the greater of the road and tire velocities. A modest amount of slip is desirable to produce tractive forces, but excessive slip prevents the tire from producing lateral forces and adequate longitudinal forces.
A wheel is said to “slip” or “spin” when the wheel exhibits a slip ratio approaching one. This occurs when a positive (accelerating) torque is applied to the wheel, and the edge of the tire has a higher velocity than the vehicle. Traction control, for example, is a known vehicle system that is designed to limit wheel spin. Conversely, a phenomenon called “locking” occurs when a negative (braking) torque is applied to a wheel. In locking, the outside edge of the tire has a lower velocity than the vehicle (e.g., the wheel stops completely relative to the vehicle), and the slip ratio approaches negative one. Anti-lock Braking Systems (ABS) are examples of vehicle systems designed to eliminate wheel locking.
Conventional motor vehicles are often driven by a single power source connected to the drive wheels by a transmission and differential. This arrangement has the marked disadvantage that if one drive wheel is on a slippery surface, the vehicle may be immobilized as the differential sends all power to the wheel least able to transmit power to the road. One way engineers have dealt with this problem is to limit the difference in speed between the two drive wheels, either through limited-slip differentials that mechanically limit the speed difference between the half-shafts to the two drive wheels, or through a brake system that brakes a slipping wheel (a technique often referred to as traction control).
If both drive wheels are on a slippery surface, however, such techniques have limited utility. This problem led to the development of four-wheel drive (4WD) and all-wheel drive (AWD) vehicles. In a 4WD vehicle, the differentials are locked, forcing all wheels to rotate at the same speed. While very effective at propelling the vehicle under slippery conditions, 4WD vehicle systems force wheels to skid around corners on dry roads, make the vehicle very difficult to drive in high-traction conditions. AWD vehicles use any of a number of systems, including the limited-slip differentials and traction control systems mentioned above, to limit wheel slip enough to maintain traction but without impeding cornering. These vehicle systems have enjoyed increasing popularity in recent years.
Nonetheless, these systems have a number of problems. In one example, the task of splitting power from one source to four drive wheels in an optimal fashion is very difficult to do mechanically. The problem is further complicated because the “optimal fashion” constantly changes with (a) varying road conditions and (b) the instantaneous weight distribution on the four wheels. The prior art has attempted to introduce mechanical devices to facilitate proper torque distribution, but such devices are heavy and complex, difficult to manufacture, subject to mechanical wear, and inefficiently waste fuel and power from the drive system. U.S. Pat. Nos. 5,850,616 and 6,208,929 describe exemplary AWD systems that are subject to limitations associated with distributing power from one power source to four wheels.
Certain vehicles, either with or without AWD, employ yaw moment control (YMC) systems. YMC systems are designed to produce a yaw moment that supplements the moment normally produced by vehicle tires by applying either positive or negative torques to selected wheels. The resulting yaw moment gives the vehicle increased turning ability in emergency situations and can correct understeer or oversteer conditions. Understeer refers to a situation where a vehicle does not turn as quickly as the driver intends due to insufficient lateral force in the front wheels; oversteer refers to a situation where a vehicle turns more quickly than the driver intends due to insufficient lateral force in the rear wheels. Current YMC systems suffer from a number of limitations. For example, YMC systems function primarily by applying a negative, or braking, torque to selected wheels, and as such, are very limited in their ability to make use of positive, or driving torque. This substantially limits the maximum yaw moment that is generated under a number of circumstances and prevents use in situations where the driver wishes to accelerate rather than decelerate.
The prior art algorithms used to determine braking torque at each wheel have similar limitations. Less advanced systems rely on lookup tables to dictate vehicle response. Such tables reduce vehicle response characteristics to a small number of predetermined possibilities that are not therefore tailored to actual and real-time driving conditions. Other systems rely on complex real-time simulation of vehicle parameters, such as the road coefficient of friction (μ) and the wheel slip angle (α). These simulations require substantial computing power and do not account for situations where model assumptions are violated or the underlying vehicle dynamics are incomplete.
A further disadvantage of both traction control and YMC systems as currently implemented is that these systems waste energy. Traction control and YMC systems generally rely on friction brakes, which turn kinetic energy to heat. This heat is radiated to the atmosphere and is not reused by the vehicle.
Advances in electric drive technologies have increased the options in designing automotive drive systems. Certain hybrid vehicle systems that use a generator in combination with an internal combustion engine to produce electricity, and certain fuel cell systems generate electrical energy directly. This electrical energy may be used to power an electric motor that provides traction for the vehicle. The torque output of such motors may be directly controlled; the motors may also be used as generators that brake the vehicle by transforming kinetic energy to electrical energy, which may then be re-used to increase overall efficiency. Further efficiency gains may be made because electrically-driven vehicles do not generally require multi-speed transmissions, thereby eliminating one source of power loss and reducing complexity in the drive train.
While electrically-driven vehicles hold considerable promise, prior art designs have often been incremental; for example, electric-drive vehicles use the same drive configuration as vehicles with internal combustion engines, by substituting the electric motor in place of the internal combustion engine. In another example, power is still often transmitted to the wheels of an electric-drive vehicle by a differential, which is still subject to the flaws discussed above. Moreover, only the braking power from two drive wheels is captured regeneratively while the other two wheels still utilize friction brakes.
The prior art has employed separate electric motors to power each drive wheel on a vehicle. For example, one High Mobility Multipurpose Wheeled Vehicle (HMMWV, or Hummvee) uses individually-controlled drive motors to achieve high performance in off-road situations. Traction control routines have also been developed for an electric snow car used on steep, snowy roads at a ski resort. These applications do not, however, utilize vehicle dynamics in algorithmic solutions.
Certain other prior art distributes torque through use of independently-controlled motors. U.S. Pat. No. 5,343,971 proposes one driving arrangement based upon multiple individually-controlled electric motors; the '971 patent however provides little support as to how the system would function in actual driving conditions. By way of example, it leaves unresolved the issue of how to prevent controller action from inducing wheel slip. Increasing the torque assignment on a wheel that already has a relatively high slip value may cause the wheel to enter a slipping state, which adversely affects the vehicle's ability to follow an intended path.
U.S. Pat. No. 5,376,868 includes YMC but bases this control on coarse look-up tables. Such a scheme is not able to respond optimally to a wide range of driving conditions.
U.S. Pat. No. 5,465,806 features independently-controlled motors for each wheel and specifies how each wheel is independently steered. The '806 patent does not allow for YMC. Further, the addition of four wheel steering adds significant complexity and requires a departure from proven vehicle suspension designs. In one example, it has no provision for re-distributing torque away from a locked or spinning wheel.
U.S. Pat. Nos. 5,164,903, 5,148,883 and 6,422,333 describe techniques to distribute torque to different wheels based on loading. U.S. Pat. No. 5,508,924 implements traction control on a vehicle driven with four independently-controlled electric motors based on estimated wheel slip values. As above, such prior art does not utilize vehicle cornering dynamics or YMC.
Certain other prior art involving a vehicle with a plurality of motors focuses on efficiency gain rather than on improvements to vehicle dynamics, for example as described in U.S. Pat. Nos. 5,879,265, 5,453,930 and 5,549,172.
While the prior art has sought to improve vehicle handling through optimal torque distributions and the electric motor, known solutions are sub-optimal from with respect to vehicle dynamics and/or vehicle efficiency.